Fix Windows script execution when called from script hooks

Console applications under Windows, such as batch scripts, require the
CREATE_NO_WINDOW process flag when run without an actual console window
present. This change allows such scripts to execute and impact the hook
status by way of their return code.

Fixes bug #240.

Signed-off-by: Josh Cepek <josh.cepek@usa.net>
Acked-by: Heiko Hund <heiko.hund@sophos.com>
Message-Id: <kn2pao$5sv$1@ger.gmane.org>
URL: http://article.gmane.org/gmane.network.openvpn.devel/7594
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit a19e35a95bf4a0177ae115535a3755d3acd894e9)